Low‐Potential Detection of Glucose with a Biosensor Based on the Immobilization of Glucose Oxidase on Polymer/Manganese Oxide Layered Nanocomposite

A nanocomposite with poly(diallyldimethylammonium), PDDA, intercalated between manganese oxide layers is constructed on a graphite electrode surface through one‐step electrodeposition and used to adsorb glucose oxidase (GOD). The immobilized GOD displays a pair of stable and quasireversible redox peaks with a formal potential of ?468 mV in pH 7.0 buffer solutions and exhibits excellent electrocatalysis to the reduction of oxygen. In the presence of dissolved oxygen, the reduction peak current decreased gradually with the addition of glucose, indicating that the immobilized GOD kept its bioactivity. Thus a reagentless biosensor for glucose at a low detection potential was established. The linear concentration range is from 0.02 to 2.78 mM with a detection limit of 9.8 μM. The proposed glucose biosensor was insensitive to common interferences such as ascorbic and uric acids etc.     

Ions Can Move a Proton‐Coupled Electron‐Transfer Reaction into Tunneling Regime

The effects of charged species on proton‐coupled electron‐transfer (PCET) reaction should be of significance for understanding/application of important chemical and biological PCET systems. Such species can be found in proximity of activated complex in a PCET reaction, although they are not involved in the charge transfer process. Reported here is the first study of the above‐mentioned effects. Here, the effects of Na⁺, K⁺, Li⁺, Ca²⁺, Mg²⁺, and Me₄N⁺ observed in PCET reaction of ascorbate monoanions with hexacyanoferrate(III) ions in H₂O reveal that, in presence of ions, this over‐the‐barrier reaction entered into tunneling regime. The observations are: a) dependence of the rate constant on the cation concentration, where the rate constant is 71 (at I = 0.0023), and 821 (at 0.5M K⁺), 847 (at 1.0M Na⁺), and 438 M ^?1 s^?1 (at 0.011M Ca²⁺); b) changes of kinetic isotope effect (KIE) in the presence of ions, where k_H/k_D=4.6 (at I = 0.0023), and 3.4 (in the presence of 0.5M K⁺), 3.3 (at 1.0M Na⁺), 3.9 (at 0.001M Ca²⁺), and 3.9 (at 0.001M Mg²⁺), respectively; c) the isotope effects on Arrhenius pre‐factor where A_H/A_D=0.97 (0.15) in absence of ions, and 2.29 (0.60) (at 0.5M Na⁺), 1.77 (0.29) (at 1.0M Na⁺), 1.61 (0.25) (at 0.5M K⁺), 0.42 (0.16) (at 0.001M Ca²⁺) and 0.16 (0.19) (at 0.001M Mg²⁺); d) isotope differences in the enthalpies of activation in H₂O and in D₂O, where ΔΔH^?(D,H)=3.9 (0.4) kJ mol^?1 in the absence of cations, 1.3 (0.6) at 0.5M Na⁺, 1.8 (0.4) at 0.5M K⁺, 1.5 (0.4) at 1.0M Na⁺, 5.5 (0.9) (at 0.001M Ca²⁺), and 7.9 (2.8) (at 0.001M Mg²⁺) kJ mol^?1; e) nonlinear proton inventory in reaction. In the H₂O/dioxane 1 : 1, the observed KIE is 7.8 and 4.4 in the absence and in the presence of 0.1M K⁺, respectively, and A_H/A_D=0.14 (0.03). The changes when cations are present in the reaction are explained in terms of termolecular encounter complex consisting of redox partners, and the cation where the cation can be found in a near proximity of the reaction‐activated complex thus influencing the proton/electron double tunneling event in the PCET process. A molecule of H₂O is involved in the transition state. The resulting ‘configuration’ is more ‘rigid’ and more appropriate for efficient tunneling with Na⁺ or K⁺ (extensive tunneling observed), i.e., there is more precise organized H transfer coordinate than in the case of Ca²⁺ and Mg²⁺ (moderate tunneling observed) in the reaction.     

Direct Electron Transfer of Glucose Oxidase and Glucose Biosensor Based on Nano-structural Attapulgite Clay Matrix

The direct electrochemistry of glucose oxidase (GOD) was achieved based on the immobilization of GOD on a natural nano‐structural attapulgite (ATP) clay film modified glassy carbon (GC) electrode. The immobilized GOD displayed a pair of well‐defined quasi‐reversible redox peaks with a formal potential (E^0′) of ?457.5 mV (vs. SCE) in 0.1 mol·L^?1 pH 7.0 phosphate buffer solution. The peak current was linearly dependent on the scan rate, indicating that the direct electrochemistry of GOD in that case was a surface‐controlled process. The immobilized glucose oxidase could retain bioactivity and catalyze the oxidation of glucose in the presence of ferrocene monocarboxylic acid (FMCA) as a mediator with the apparent Michaelis‐Menten constant K^app_m of 1.16 mmol·L^?1. The electrocatalytic response showed a linear dependence on the glucose concentration ranging widely from 5.0×10^?6 to 6.0×10^?4 mol·L^?1 (with correlation coefficient of 0.9960). This work demonstrated that the nano‐structural attapulgite clay was a good candidate material for the direct electrochemistry of the redox‐active enzyme and the construction of the related enzyme biosensors. The proposed biosensors were applied to determine the glucose in blood and urine samples with satisfactory results.     

Direct Electron Transfer and Electrocatalysis of Hemoglobin on Chitosan‐TiO2 Nanorods‐Glass Carbon Electrode

The direct electron transfer between hemoglobin (Hb) and the glassy carbon electrode (GC) can be readily achieved via a high biocompatible composite system based on biopolymer chitosan (CHT) and TiO₂ nanorods (TiO₂‐NRs). TiO₂‐NRs greatly promote the electron transfer between Hb and GC, which contribute to the higher redox peaks. UV‐vis spectra result indicated the Hb entrapped in the composite film well keep its native structure. The immobilized Hb remains its bioelectrocatalytical activity to the reduction of H₂O₂ with a lower detection limit. A novel, sensitive, reproducible and stable electrochemical biosensing platform of H₂O₂ based on Hb‐TiO₂‐CHT electrode is explored.     

On the Hopping Efficiency of Nanoparticles in the Electron Transfer across Self‐Assembled Monolayers

Redox reactions of solvated molecular species at gold‐electrode surfaces modified by electrochemically inactive self‐assembled molecular monolayers (SAMs) are found to be activated by introducing Au nanoparticles (NPs) covalently bound to the SAM to form a reactive Au–alkanedithiol–NP–molecule hybrid entity. The NP appears to relay long‐range electron transfer (ET) so that the rate of the redox reaction may be as efficient as directly on a bare Au electrode, even though the ET distance is increased by several nanometers. In this study, we have employed a fast redox reaction of surface‐confined 6‐(ferrocenyl) hexanethiol molecules and NPs of Au, Pt and Pd to address the dependence of the rate of ET through the hybrid on the particular NP metal. Cyclic voltammograms show an increasing difference in the peak‐to‐peak separation for NPs in the order Au<Pt<Pd, especially when the length of the alkanedithiol increases from octanedithiol to decanedithiol. The corresponding apparent rate constants, k_app, for decanedithiol are 1170, 360 and 14 s^?1 for NPs of Au, Pt and Pd, respectively, indicating that the efficiency of NP mediation of the ET clearly depends on the nature of the NP. Based on a preliminary analysis rooted in interfacial electrochemical ET theory, combined with a simplified two‐step view of the NP coupling to the electrode and the molecule, this observation is referred to the density of electronic states of the NPs, reflected in a broadening of the molecular electron/NP bridge group levels and energy‐gap differences between the Fermi levels of the different metals.     

Enhanced Direct Electron Transfer of a Multihemic Nitrite Reductase on Single‐walled Carbon Nanotube Modified Electrodes

Single‐walled carbon nanotubes (SWCNTs) deposits on glassy carbon and pyrolytic graphite electrodes have dramatically enhanced the direct electron transfer of the multihemic nitrite reductase from Desulfovibrio desulfuricans ATCC 27774, enabling a 10‐fold increase in catalytic currents. At optimal conditions, the sensitivity to nitrite and the maximum current density were 2.4±0.1 A L mol^?1 cm^?2 and 1500 µA cm^?2, respectively. Since the biosensor performance decreased over time, laponite clay and electropolymerized amphiphilic pyrrole were tested as protecting layers. Both coating materials increased substantially the bioelectrode stability, which kept about 90 % and 60 % of its initial sensitivity to nitrite after 20 and 248 days, respectively.     

Electrochemical Behavior of Glucose Oxidase Immobilized on Pd‐Nanoparticles Decorated Ionic Liquid Derived Fibrillated Mesoporous Carbon

Pd nanoparticles with an average diameter of 5 nm were decorated on the surface of ionic liquid derived fibrillated mesoporous carbon (IFMC) to prepare a novel nano‐hybrid material (Pd@IFMC). Thereafter, glucose oxidase was immobilized on Pd@IFMC modified glassy carbon electrode to fabricate an enzymatic glucose biosensor. A pair of well‐defined redox peaks was recorded for direct electron transfer of the immobilized glucose oxidase at the formal potential of ? 0.418 V with a peak to peak separation of 25 mV. Electron transfer rate constant of was calculated to be 14.6 s^?1. The response of fabricated biosensor was linear towards glucose concentration.     

Reversible Switching of Redox‐Active Molecular Orbitals and Electron Transfer Pathways in CuA Sites of Cytochrome c Oxidase

The Cu_A site of cytochrome c oxidase is a redox hub that participates in rapid electron transfer at low driving forces with two redox cofactors in nearly perpendicular orientations. Spectroscopic and electrochemical characterizations performed on first and second‐sphere mutants have allowed us to experimentally detect the reversible switching between two alternative electronic states that confer different directionalities to the redox reaction. Specifically, the M160H variant of a native Cu_A shows a reversible pH transition that allows to functionally probe both states in the same protein species. Alternation between states exerts a dramatic impact on the kinetic redox parameters, thereby suggesting this effect as the mechanism underlying the efficiency and directionality of Cu_A electron transfer in vivo. These findings may also prove useful for the development of molecular electronics.     

On‐line Concentration and Separation of Parabens by Micellar Electrokinetic Chromatography Using Polymer Solutions

Simultaneous separation and quantification of seven parabens commonly used as preservatives in cosmetic products, by micellar electrokinetic chromatography with a stacking technique has been demonstrated. An effective on‐line concentration strategy involving a combination of sweeping and the use of polymer solutions is a key feature of the proposed method, which successfully determined individual parabens. The analysis parameters such as injection time, pH and concentration of phosphate solution, and concentration of sodium dodecyl sulphate (SDS) and poly(ethylene oxide) (PEO) were examined. The optimum conditions were found to be as follows: a 15 mM phosphate solution (pH 9.5) containing 20 mM SDS for filling the capillary, and for the separation electrolytes, 0.100% PEO (8 MDa) added to the phosphate and SDS solution of the same composition as for the capillary. The entire analysis process was completed in 13 min and a 930‐2200‐fold enhancement factor was achieved. The LODs (S/N = 3) for this approach were in the range from 4.32 to 7.78 nM. The linear range for each paraben was between 50 nM and 5.0 μM (R² > 0.990). The optimized method was then successfully applied to the determination of parabens in commercial cosmetic products.     

Electrochemical Properties of Ordered Mesoporous Carbon Film Adsorbed onto a Self‐Assembled Alkanethiol Monolayer on Gold Electrode

A stable ordered mesoporous carbon (OMC) film electrode was successfully constructed by adsorbing OMC onto a self‐assembled monolayer (SAM) of C₁₈H₃₇SH chemisorbed on the Au electrode. Transmission electron microscopy (TEM), atomic force microscopy (AFM), and electrochemical methods were used to characterize the properties of the OMC film electrode. The adsorbed OMC can restore the heterogeneous electron transfer almost totally blocked by the alkanethiol monolayer. Nyquist plots show a sharply decrease of the charge transfer resistance (R_ct) of the Fe(CN) couple at the OMC film electrode. Furthermore, the OMC film electrode is found to possess a significantly reduced interfacial capacitance and largely enhanced current response of hydrogen peroxide. This novel approach to the fabrication of stable OMC film electrode with excellent electrochemical properties is believed to be very attractive for electrochemical studies and electroanalytical applications.     

Direct Electrochemistry of Glucose Oxidase Immobilized on Titanium Carbide‐Au Nanoparticles‐Fullerene C60 Composite Film and Its Biosensing for Glucose

The direct electrochemistry of glucose oxidase (GOD) immobilized on the designed titanium carbide‐Au nanoparticles‐fullerene C₆₀ composite film modified glassy carbon electrode (TiC‐AuNPs‐C₆₀/GCE) and its biosensing for glucose were investigated. UV‐visible and Fourier‐transform infrared spectra of the resulting GOD/TiC‐AuNPs‐C₆₀ composite film suggested that the immobilized GOD retained its original structure. The direct electron transfer behaviors of immobilized GOD at the GOD/TiC‐AuNPs‐C₆₀/GCE were investigated by cyclic voltammetry in which a pair of well‐defined, quasi‐reversible redox peaks with the formal potential (E^0′) of ‐0.484 V (vs. SCE) in phosphate buffer solution (0.05 M, pH 7.0) at the scan rate of 100 mV·s^?1 were obtained. The proposed GOD modified electrode exhibited an excellent electrocatalytic activity to the reduction of glucose, and the currents of glucose reduction peak were linearly related to glucose concentration in a wider linearity range from 5.0 × 10^?6 to 1.6 × 10^?4 M with a correlation coefficient of 0.9965 and a detection limit of 2.0 × 10^?6 M (S/N = 3). The sensitivity and the apparent Michaelis‐Menten constant (K_M^app) were determined to be 149.3 μA·mM^?1·cm^?2 and 6.2 × 10^?5 M, respectively. Thus, the protocol will have potential application in studying the electron transfer of enzyme and the design of novel electrochemical biosensors.     

Direct Voltammetry of Copper,Zinc‐Superoxide Dismutase Immobilized onto Electrodeposited Nickel Oxide Nanoparticles: Fabrication of Amperometric Superoxide Biosensor

Direct electron transfer of immobilized copper, zinc‐superoxide dismutase (SOD) onto electrodeposited nickel‐oxide (NiOx) nanoparticle modified glassy carbon (GC) electrode displays a well defined redox process with formal potential of ?0.03 V in pH 7.4. Cyclic voltammetry was used for deposition of (NiOx) nanoparticles and immobilization of SOD onto GC electrode. The surface coverage (Γ) and heterogeneous electron transfer rate constant (k_s) of immobilized SOD are 1.75×10^?11 mol cm^?2 and 7.5±0.5 s^?1, respectively. The biosensor shows a fast amperometric response (3 s) toward superoxide at a wide concentration range from 10 µM to 0.25 mM with sensitivity of 13.40 nA µM^?1 cm^?2 and 12.40 nA µM^?1 cm^?2, detection limit of 2.66 and 3.1 µM based on anodically and cathodically detection. This biosensor exhibits excellent stability, reproducibility and long life time.     

Electron transfer of heme proteins on the ITO electrode in polymer solvents

The redox reaction of poly(ethylene oxide) (PEO)-modified hemoglobin (PEO–Hb) was analyzed in PEO oligomers with cyclic voltammetry. The PEO–Hb was made soluble in PEO with molecular weight of 200 (PEO₂₀₀) containing 0.5 M KCI. Quasi-reversible redox signals of PEO–Hb were obtained by using an indium tin oxide (ITO) glass working electrode. PEO–Hb, cast on the ITO electrode, also showed the redox response in PEO with molecular weight of 400 (PEO₄₀₀). The peak current of PEO–Hb on the ITO electrode gradually increased during potential cycling. The effect of the scan rate on the quantity of electricity (Q) was analyzed after the peak current reached a constant value. The constant Q value was observed at the scan rate ranging from 30 to 500 mV/sec. The number of reactive PEO–Hb molecules was estimated from this constant Q-value. It was suggested that the electron transfer was carried out at the first layer of the PEO–Hb which was in direct contact with the ITO electrode. The quantity of electricity of PEO–Hb increased when the ITO electrode was first washed in an aqueous medium with ultrasonicator. This strongly suggested that the more effective surface area of the ITO electrode turned to be covered with PEO–Hb when the microporous region of the ITO particles was more hydrated.     

Direct Electrochemistry of Glucose Oxidase Immobilized at a Novel Composite Material β‐Cyclodextrin/poly(4‐aminothiophenol)/Au Nanoparticle Modified Electrode

A novel complex material was fabricated by three steps. In the first step, gold nanoparticle (Au_nano) was prepared with the method of chemistry and dialysis. In the second step, 4‐aminothiophenol (AT) was encapsulated in the cavity of β‐cyclodextrin and formed inclusion complex, cyclodextrin/4‐aminothiophenol (CD/AT). And then inclusion complex was adsorbed to the surface of Au_nano based on the bond of Au‐S interaction. In the last step, a complex material, cyclodextrin/poly(4‐aminothiophenol)‐Au nanoparticles (CD/PAT‐Au_nano) was obtained by the polymerizing in the acid solution initiated by chlorauric acid. The CD/PAT‐Au_nano has spherical nanostructure with the average diameter of 55 nm. Glucose oxidase (GOx) was anchored with this complex material and direct electrochemistry of GOx was achieved. A couple of stable and well‐defined redox peaks were observed with the formal potential (E^0′) of ‐0.488 V (vs. SCE) in a pH 6.98 buffer solution. The GOx modified electrode also exhibited an excellent electrocatalytic activity to the reduction of glucose, a linearity range for determination of glucose is from 0.25 mM to 16.0 mM with a detection limit of 0.09 mM (S/N = 3). This protocol had potential application to fabricate the third‐generation biosensor.     

Synthesis of H‐shaped A3BA3 copolymer by methyl‐2‐nitrosopropane induced single electron transfer nitroxide radical coupling

Amphiphilic H‐shaped [poly(ethylene oxide)]₃‐polystyrene‐[poly(ethylene oxide)]₃(PEO₃‐PS‐PEO₃) copolymer was synthesized by 2‐methyl‐2‐nitrosopropane (MNP) induced single electron transfer nitroxide radical coupling (SETNRC) using PEO₃‐(PS‐Br) as a single precursor. First, the A₃B star‐shaped precursor PEO₃‐(PS‐Br) was synthesized by atom transfer radical polymerization (ATRP) using three‐arm star‐shaped PEO₃‐Br as macro‐initiator. Then, in the presence of Cu(I)Br/Me₆TREN, the bromide group at PS end was sequentially transferred into carbon‐centered radical by single electron transfer and then nitroxide radical by reacting with MNP in mixed solvents of dimethyl sulfoxide (DMSO)/tetrahydrofuran (THF), and in situ generated nitroxide radical could again capture another carbon‐centered radical by fast SETNRC to form target PEO₃‐PS‐PEO₃ copolymer. The MNP induced SETNRC could reach to a high efficiency of 90% within 60 min. After the product PEO₃‐PS‐PEO₃ was cleaved by ascorbic acid, the SEC results showed that there was about 30% fraction of product formed by single electron transfer radical coupling (SETRC) between carbon‐centered radicals. © 2011 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2011     

Switchover of the Mechanism between Electron Transfer and Hydrogen‐Atom Transfer for a Protonated Manganese(IV)–Oxo Complex by Changing Only the Reaction Temperature

Hydroxylation of mesitylene by a nonheme manganese(IV)–oxo complex, [(N4Py)Mn^IV(O)]²⁺ ( 1 ), proceeds via one‐step hydrogen‐atom transfer (HAT) with a large deuterium kinetic isotope effect (KIE) of 3.2(3) at 293 K. In contrast, the same reaction with a triflic acid‐bound manganese(IV)‐oxo complex, [(N4Py)Mn^IV(O)]²⁺‐(HOTf)₂ ( 2 ), proceeds via electron transfer (ET) with no KIE at 293 K. Interestingly, when the reaction temperature is lowered to less than 263 K in the reaction of 2 , however, the mechanism changes again from ET to HAT with a large KIE of 2.9(3). Such a switchover of the reaction mechanism from ET to HAT is shown to occur by changing only temperature in the boundary region between ET and HAT pathways when the driving force of ET from toluene derivatives to 2 is around ?0.5 eV. The present results provide a valuable and general guide to predict a switchover of the reaction mechanism from ET to the others, including HAT.